[r] Allow writing raw as metadata for groups and arrays

[mojaveazure]

The R API errors out when trying to write a raw as array- or group-level meta data; this should be allowed as raw is equivalent to Python bytes

> uri <- tempfile("r-array-meta")
> arr <- SOMASparseNDArrayCreate(uri, type = arrow::int32(), shape = c(200L, 100L))
> arr$set_metadata(list(str = "test_character"))
> arr$set_metadata(list(bytes = charToRaw("test_raw")))
Error: Unsupported type 'raw'